Method of metallurgically bonding a internally finned heat exchange structure

ABSTRACT

A METHOD OF ACHIEVING GOOD THERMAL INTERCHANGE BETWEEN A TUBE WALL AND A TUBE INSTALLED FIN ANNULUS, PROVIDING SECONDARY HEAT TRANSFER SURFACE, WHEREIN FIN CORRUGATIONS ARE METALLURGICALLY BONDED TO THE TUBE WALL BY A METHOD INHIBITING THE BONDING MATERIAL FROM FLOWING TO AND BLOCKING FLOW PATHS BETWEEN THE FIN CORRUGATIONS. A NEW FIN MATERIAL IS USED WHICH MAINTAINS OPEN FLOW PATHS THROUGH THE FIN MATERIAL EVEN WHEN THE MATERIAL IN STRIP FORM IS ROLLED TO A CIRCULAR OR ARCUATE CONFIGURATION. D R A W I N G

United States Patent 1 1 Degroote [45] Aug. 27, 1974 [54] METHOD OFMETALLURGICALLY 3,359,616 12/1967 Butt 2911513 A BONDING A INTERNALLYFINNED HEAT 1,651,403 12/1927 Mougey- EXCHANGE STRUCTURE 2,703,9213/1955 Brown 29/157.3 A

Inventor: Raymond S. Degroote, Centeryille,

Ohio

Assignee: United Aircraft Products, Inc.,

Dayton. Ohio Filed: Nov. 22, 1971 Appl. No: 201,003

US. Cl 29/l57.3 A, 29/490, 29/DIG. 4, 113/118 A Int. Cl 821d 53/02 Fieldof Search 29/157.3 R, 157.3 A, 490, 29/D1G. 4; 113/118 R, 118 A PrimaryExaminer-Charles W. Lanham Assistant Examiner-D. C. Reiley, IIIAttorney, Agent, or Firm-J. E. Beringer [5 7] ABSTRACT A method ofachieving good thermal interchange between a tube wall and a tubeinstalled fin annulus, providing secondary heat transfer surface,wherein fin corrugations are metallurgically bonded to the tube wall bya method inhibiting the bonding material from flowing to and blockingflow paths between the fin corrugations. A new fin material is usedwhich maintains open flow paths through the fin material even when thematerial in strip form is rolled to a circular or arcuate configuration.

8 Claims, 7 Drawing Figures PATENTED Mme-1 m BACKGROUND OF THE INVENTIONThis invention relates to the heat transfer art and particularly to amethod of fabrication of a tube assembly comprising a tube and aninterior fin means. The invention further comprehends a fin meansuniquely configured to establish and maintain open flow area between fincorrugations when the fin means is rolled and installed in a tube.

Tube assemblies of the kind to which the invention pertains comprise atube usually arranged in a bundle with other like tubes for a flow of afirst fluid over and around the tube exteriors. A second fluid,different in temperature from that of the first and separated therefrom,is passed through the tubes. An exchange of heat occurs through the tubewalls. A strip of corrugated metal, termed a fin, is contoured or bentto a circular shape and inserted in the tube where it is in commoncontact with the tube wall and the internally flowing second fluid. Thefin is an extended surface member, supplementing the tube wall inachieving a conduct of heat. It has an efficiency determined in largepart by the excellence and continuity of its contact with the tube wall.A high degree of heat transfer efficiency is something to be desired inany heat exchanger design but in compact high performance devices issought with particular diligence.

In the prior art various methods have been proposed for accomplishinghigh level heat transfer between the tube wall and fin. Among the moreattractive of the proposed methods is metallurgical bonding wherein thecrests of the corrugated fin join unitarily to the tube wall through asolder or braze material. According to this concept flowable bondingmaterial would inher ently fill any gaps existing between the fin crestsand the tube wall, and, ideally, there would be continuous metal tometal contact between the tube and fin. In principle, therefore,metallurgical bonding is an optimal solution to the problem ofminimizing contact resistance, that is, resistance to a free flow ofthermal energy from the fin to the tube wall, or vice versa.

In practical terms, however, metallurgical bonding has been somethingless than an unqualified success and is largely unknown in compact highperformance heat exchangers. The reasons for this are various. Cost isfrequently a factor. For example to apply the solder or braze materialit has been suggested that the fin strip or the tube wall, or both, beplated. In most instances the cost of this process offsets theadvantages resulting from metallurgical bonding. In another method, itis proposed to feed molten solder into one end of a tube in which a finannulus has previously been installed. Capillary and other influencesare relied upon to position the solder uniformly along the fin crests.Inconsistency obviously inheres in this process, however, with voidsoccurring in random fashion in the defined metallurgical joints. Fromengineering and production standpoints it is highly desirable thatfinished tubes and finished heat exchangers made from such tubes performto predictable, uniform standards. Such performance is beyond thecapabilities of the latter and other known methods practiced orexperimented with.

A common denominator in the deficiencies of prior art methods is aproblem of plugging of flow passages between fin corrugations caused bymolten solder running into the spaces defining these passages andhardening therein. To reduce the flow area of the fin passages is toincrease pressure drop, to reduce effective heat transfer area andotherwise adversely to affect the ability of the heat exchanger to carryout its intended function.

The fin strip which forms the fin annulus is made to be more or lessdense in terms of the number of fin corrugations per unit of measure. Amore dense fin provides greater heat transfer surface but the passagestherethrough are more narrow and hence more easily plugged. Moreover theproblem of closing or partial closing of fin passages exists apart fromplugging by molten solder. A strip rolled to a circular configurationhas an inner and an outer periphery of differing diame tral dimensions.At and near the inner periphery the fin corrugations are forced closertogether, resulting in reduced flow area and a compromised ability tofunction to design criteria. In the absence of any means to overcomethis condition, larger tubes or fins of less dense material must be usedresulting in either case in a heat exchanger larger and more expensivethan is desirable.

SUMMARY OF THE INVENTION The present invention has in viewa method ofmaking a heat transfer tube or the like in which a flowable metallicbonding material is placed at the interface be tween the interior tubewall and an inserted fin annulus, the material being normally in asubstantially nonflowing condition. The tube assembly comprising thetube and installed fin annulus is heated to a temperature value toreduce the metallic bonding material to a flowable condition. The tubeassembly is subsequently cooled below the described temperature valuewhereby the metallic material solidifies as a bond between the finannulus and the tube wall. Provision is made substantially to restrictthe bonding material to the point of engagement of the fin crests withthe tube wall to avoid plugging of spaces between the fin corrugations.For example, during at least a portion of the cooling step of theprocess the tube assembly may be rotated relatively rapidly about itslongitudinal axis. Centrifugal forces are utilized thereby to inhibitflow of the flowable material between the tin corrugations. According toa further aspect of the invention the fin strip material is constructedof flexible strip material crimped to define a plurality of relativelyclosely spaced continuous fin convolutions or corrugations. Each suchcorrugation comprises approximately vertical walls interconnecting upperand lower crests which are alternately broad and narrow to formalternately wide and narrow spaces between adjacent fin walls with widespaces opening to one side of the strip material and narrow spacesopening to the other side. In re sponse to flexing of the strip in aselected direction about an axis parallel to the fin corrugations therelatively wider spaces tend to close and the more narrow spaces tend toopen whereby to maintain substantially uniform open flow passagesthrough the fin material.

An object of the invention is to provide an improved methodsubstantially of the class described in which advantages of themetallurgical bonding concept in tube and fin construction are realizedwhile obviating problems inhering therein in the prior art.

Another object of the invention is to provide fin means of particularutility in avoiding a reduction of flow area in a formed fin, as forexample a fin formed to a circular shape and installed in a heatexchange tube.

Other objects and structural details of the invention will appear fromthe following description, when read in connection with accompanyingdrawings, wherein:

FIG. 1 is a view in side elevation of a tube assembly produced inaccordance with the instant invention, one end of the tube being brokenaway to show the fin material installed therein;

FIG. 2 is an end view of the tube assembly of FIG. 1;

FIG. 3 is a relatively enlarged fragmentary view showing the solderedjoint as defined at the interface of individual fin convolutions and thetube wall;

FIG. 4 is a partly diagrammatic view of apparatus useful in a rotatingof the tube assembly;

FIG. 5 is a view in perspective of a fin strip from which the finannulus is formed;

FIG. 6 is a fragmentary end view of the fin strip as it appears prior torolling to a circular form; and

FIG. 7 is a view similar to FIG. 6 showing the fin material as itappears in a rolled or formed condition.

Referring. to the drawing, a tube assembly as achieved by the method ofthe invention includes an outer tube 10 made ofa metal of good thermalconductivity. The tube 10 is ofa uniform diameter, is relatively thinwalled and is open at both ends. There may be comprised in the tubeassembly an inner tube 11 which may be constructed like the tube 10 butwhich in any event provides with the outer tube an annular space to beoccupied by a rolled strip 12 of thin material. The strip 12 iscomprised of a thin gauge, ductile metal of good thermal conductivity.Originally in sheet form it is gathered and crimped to a corrugatedformation to define a series of parallel fin convolutions of longitudinal extent. Each convolution or corrugation comprises upper and lowercrests in the form ofa peak portion 13 and a valley portion 14 connectedby approximately vertical or radial wall portions 15. The fincorrugations are in the illustrated instance straight but may assume awave-like configuration or any of the other shapes to which material ofthe present class conventionally is formed.

The method ofthe invention may involve a preassembly ofthe parts inwhich fin strip 12 is rolled to an annular configuration about innertube 11 with these parts then being inserted as a sub-assembly intoouter tube 10. With the parts so positioned, the strip 12 has an outersurface presented for contact with the inner wall of tube 10 and aninner surface presented for contact with the exterior of inner tube 11.The parts will preferably have a friction fit in which they areyieldingly held in an assembled relation. If found necessary ordesirable to achieve a more uniformly contacting relation of the finmaterial to the outer tube wall the inner tube 11 may be expanded, as bypassing a mandrel therethrough. The method of the invention does notcontemplate reliance upon expansion of the inner tube 11 to minimizecontact resistance at the outer tube wall, however. but instead providessolid, metallic joints at the peak portions 13 for a direct conduct ofheat between the fin material and the tube wall.

In the practice of the metallurgical bonding concept, a solder or brazematerial is suitably applied at the interface between the peak finportions I3 and the tube wall. This may variously be done, as forexample by pre-plating the interior wall oftube It) or by pre-platingone or both sides of the strip material 12. In another possible methodthe tube 10 is dipped in a molten solder material which coats theinterior wall of the tube and when allowed to harden forms a layer ofsolder material thereon. According to still another possible method acommercially available solder composition in paste form is brushed ontothe peak portions I3 of the fin corrugations, the brushing beingpreferably done in a direction transverse to the long dimension of thefins to avoid depositing solder material in the spaces between the fins.In any event, with solder or braze material present at the interfacebetween the tube wall and the fin strip, the tube assembly is heated andbrought to a temperature approximating that at which the solder or brazematerial is reduced to a flowable condition. When this occurs thebonding material assumes an intimately contacting relation to the tubewall and to the upper fin crests and to such extent as may be requiredflows along the fin corrugations fully to occupy any voids or gaps whichmay be present between the peak portions 13 and the tube wall. Uponallowing the tube assembly to cool, the molten material solidifies toform a strong, sound joint between the tube wall and each individual fincrest providing low or substantially no resistance to heat transferbetween the tube wall and the fin material.

Installations using a formed tube assembly may include one in which aplurality of formed tubes are disposed in a bundle for flow of a firstfluid over and around the tube exteriors. A second fluid, in separatedrelation to the first is controlled and directed to pass longitudinallythrough the tubes. In accordance with the concern of the presentinvention, the flow is in an annular form, inner tube I1 being closed.The flowing fluid is in contact with the inner wall of tube 10 and isalso in contact with the material of fin strip 12. Assuming the fluidsto be of different temperature, a transfer of heat takes place throughthe tube wall. The tube wall serves as primary heat transfer surfacewith respect to the second fluid in contact therewith. The material offin strip 12 acts as a secondary surface, conducting heat to or from thetube wall. The soldered or brazed joints at the points of contact of thefin strip with the tube wall insure excellent heat transfercharacteristics.

Returning to a consideration of the fabrication method it is desirableto confine the metallic bonding material to a region at or adjacent tothe tube wall. In a molten condition the bonding material may tend tomove into and occupy a portion of the space between adjacent fincorrugations, which spaces, indicated at 16 in FIG. 2, act as flowpassages for the described second fluid which flows longitudinallythrough the tubes. If allowed to enter such spaces and subsequently tosolidify therein, the bonding material reduces both effective flow andsurface area and correspondingly diminishes the heat transfercapabilities of the provided secondary heat transfer surface. Thiscondition may be alleviated by utilizing carefully controlled quantitiesof bonding material and applying it in a restricted manner, as forexample only to the tips of peak portions 13 of the fin strip. Inanother possible method, heating of the tube assembly to the liquidustemperature of the bonding material may be effected by blowing hot airthrough the tubes, with the air temperature being gradually reduced asthe critical temperature is reached and surpassed. Air pressure wouldinherently confine the bonding material to the tube wall and would blowclear the passages between the fin corrugations.

In another contemplated method step, centrifugal force is used tomaintain the molten bonding material outwardly at the joints defined bythe tube wall and the peak portions 13 of the fin material. Inaccordance with this inventive concept, the tube assembly is heated inany desired manner to the critical temperature value, immediatelyfollowing which the tube assembly is placed in a suitable fixture androtated relatively rapidly about its longitudinal axis. The moltenmaterial, including any which may have entered the spaces between thefin corrugations is forced outwardly against the tube wall where it isconfined to a formation of the described joints. The tube may be allowedto cool naturally while rotating but in a preferred form of theinvention is simultaneously cooled by flowing air longitudinally throughthe rotating tube. In a cooled, finished tube assembly, as shown inFIGS. 1 to 3, the flow passages 16 are free of solder or braze materialwhich is present only as a thin band or fillet 17 between each peakportion 13 and a substantially contacted wall por tion of the tube 10.

Rotation of the tube assembly may be by any suitable apparatus. Forexample, there may be provided, as shown in FIG. 4, spaced apart rollersor sets of rollers 18 and 19 one of which is positively driven in arotary sense and the other of which idles or rotates freely. A carriagemember 21 slides on a table 22 toward and from the rollers 18 and 19.Bracket means 23 is mounted to the carriage 2] and supports idler rollermeans 24 in a horizontal plane passing between the rollers l8 and 19.Also mounted to the carriage 21 are forwardly projecting finger means 25terminating in upstanding extremities 26. The means 25-26 provides aholder upon which a pre-heated tube assembly is placed, in the mannerindicated, with upstanding extremities 26 holding the tube substantiallyto a contacting relation to roller means 24. As will be evident, uponadvance ofthe carriage 21 to the right, as viewed in FIG. 4, the tube 10will achieve further, simultaneous contact with the rollers 18 and 19.In such position of confinement, and due to the positive rotary drive ofone of the rollers 18 and 19, as for example roller 18, the tubeassembly is put into rotation about its longitudinal axis. There may beprovided, in a manner which it is unnecessary here to consider, acompressed air attachment by which air is blown longitudinally throughthe tube in accompaniment with its rotation.

As heretofore noted, the fin strip 12 is formed from a flat sheet ofthin ductile material with the convolutions therein being formed bysuitable die elements interacting to effect a gathering or crimping ofthe sheet material. In accordance with a feature of the invention, thedie elements are constructed to achieve differential spacing on oppositesides of the approximately vertical walls which join together upper andlower crests of a formed fin strip. Thus, and as shown in FIGS. 5 and 6,a formed fin strip comprises the before mentioned spaces or passages 16,and, intermediately thereof, relatively wider spaces 27, the formeropening through what may be considered an upper side of the tin stripand the latter opening through what may be considered the lower sidethereof. In the forming of such differential spaces, the peak portions13 of the convolutions are made relatively broad and the valley portions14 made relatively narrow. In the illustrated instance this isaccomplished by forming the peak portions to a relatively larger radiusthan the one to which the valley portions are formed. It will beunderstood, however, that the peak and valley portions are notnecessarily of an arcuate configuration. The crests may be squared offor be made more sharply angular.

In forming the fin strip about the tube 11, or in otherwise arcuatelyshaping the strip for installation in a heat transfer duct, the strip isbent about an axis substantially parallel to the long dimensions ofthefin corrugations, with the previously defined upper side of the finstrip forming the periphery oflarger radius and the described lower sideof the strip forming the periphery of smaller radius. Spaces 16accordingly open through the outer periphery of the formed strip andspaces 27 open through the inner periphery. In the forming process, asseen in FIG. 7, adjacent valley portions 14 tend to move more closelytogether while the peak portions 13 tend to separate. Correspondingadjustments in the area of the spaces 16 and 27 result and as aconsequence these areas become more :nearly alike or in any event tendto maintain their individual identities. In the absence of speciallyconfigured fin strip material a rolled fin strip will find the valleyportions thereof forced together in a manner materially to restrict flowthrough the fin annulus with consequent adverse effect upon heattransfer efficiency. In the present instance, and as shown also in FIG.2, both sets of flow passages 16 and 27 remain open in the installedposition of the fin material, permitting the fin assembly to function inproper accord with its design specifications.

The invention has been disclosed with reference to particular method andstructural embodiments. Modifications have been discussed and these andothers obvious to a person skilled in the art to which the inventionrelates are considered to be within the intent and scope of theinvention.

What is claimed is:

l. A method of metallurgically bonding a tube contained fin annulus to atube wall for maximal heat trans fer therebetween, including the stepsof applying a flowable metallic bonding material so that it is presentin a normally non-flowing condition at the interface between theinterior wall of a tube and a fin annulus installed in the tube, heatingthe tube assembly comprising the tube and installed fin annulus to atemperature value to reduce the metallic material to a flowablecondition, cooling the tube assembly below said temperature valuewhereby said metallic material solidifies as a bond between said finannulus and the tube wall, and confining the bonding material duringcooling to a region at the interface between the interior tube wall andsaid fin annulus to obviate its entrance into and plug ging of spacesbetween fin convolutions, said last named step including a rotating ofthe tube assembly relatively rapidly about its axis during at least aportion of the cooling step to utilize centrifugal force to inhibit flowof the flowable material between the fin convolutions while maintainingsuch material at the said inter face between said interior tube wall andsaid fin annulus.

2. A method according to claim ll, wherein the flowable metallic bondingmaterial is applied as a coating on the interior tube wall, with the finannulus being installed in a pre-coated tube for subsequent performanceof said heating and cooling steps.

3. A method according to claim 1, wherein the flowable metallic bondingmaterial is applied as a coating to the crests of individual tins of thefin annulus, with the pre-coated fin being installed in a tube forsubsequent performance of said heating and cooling steps.

4. A method according to claim 3, wherein said bonding material isapplied in paste form.

5. A method according to claim 4, wherein said bonding material isapplied in paste form by brushing in a direction transverse to thelongitudinal direction of the fins.

6. A method according to claim 1, wherein the tube assembly with themetallic bonding material applied thereto is heated and in a subsequentstep is rotated, with cooling being carried on simultaneously withrotation.

7. A method according to claim 1, wherein the fin annulus is comprisedof flexible strip material crimped to establish continuous finconvolutions which at peak portions thereof are relatively more broadthan at adjacent interconnecting valley portions to define alternatelywide and narrow spaces between adjacent fin walls with wide spacesopening to one side of the strip material and narrow spaces opening tothe other side.

8. A method according to claim 7, wherein in forming said fin annulussaid crimped strip is flexed to a circular configuration with the sidehaving the wide opening spaces facing radially inward.

